1. Field of the Invention
The present invention relates to a panel heater for heating a front panel which mount a print head of a hot melt ink jet printer.
2. Description of Related Art
A hot melt ink jet printer (hereinafter referred to as "a printer") includes a print head mounted on a carriage. The print head includes a heating tank, an ink tank, a plurality of nozzle heads, and heaters for heating these components. A hot melt ink (hereinafter referred to as an "ink") is in its solid state at a normal temperature and changes to its liquid state when heated. Solid ink supplied to the heating tank is heated, melted, and supplied to the ink tank and further to the nozzle head. The ink in the ink tank and the nozzle head is maintained in its liquid state. Each of the nozzle heads includes a piezoelectric member forming an ink channel. By applied with a voltage, the piezoelectric member is deformed, thereby changing internal pressure of the ink channel. As a result, ink in the ink channel is ejected as an ink droplet toward a printing medium.
When the heaters heat the ink in the ink tank and the nozzle heads to maintain the ink in its liquid state, the print head is also influenced by a various kinds of cooling factors. For example, the printer includes a rotating drum for feeding a printing medium. As the rotating drum rotates, an air current is generated between the rotating drum and the print head, thereby cooling the print head. Also, when the carriage moves back and forth, the print head mounted on the carriage loses its heat. Further, heat of the print head is radiated and transmitted through other printer components. Moreover, the print head does not lose its heat uniformly. For example, the print head is most likely loose its heat at surfaces facing to a head moving direction.
In order to overcome the above-described problems and to maintain the print head, especially the nozzle heads, at an uniform temperature, Japanese Laid-Open Patent Application No. HEI-7-17054 discloses a flexible-hybrid laminated heating device. As shown in FIG. 1(a), a heating device 290 includes a heating thin plate and a meandering heating element 300 attached thereon. The heating device 290 is divided into 11 regions 310A through 310K arranged both in a X direction and Y direction, so that uneven thermal loss can be prevented. Because an uniform current flows through the heating element 300, a wattage density of each of the regions 310A to 310K is in proportion to the resistance of the corresponding heating element 300. It should be noted that the X direction indicates the head moving direction, and the Y direction is a direction perpendicular to the X direction. In this case, resistance are set to 4.85 .OMEGA. for the regions 310A, 310K, 1.77 .OMEGA. for the regions 310B, 310J, 1.94 .OMEGA. for the regions 310C, 310I, 2.39 .OMEGA. for the regions 310D, 310F, 1.83 .OMEGA. for the region 310E, 1.81 .OMEGA. for the region 310G, and 2.54 .OMEGA. for the region 310H.
When the heating device 290 generates heat, a main surface 240 of the nozzle head increase its temperature as shown in FIG. 1(b). A thermal difference between adjacent isotherms 320 is 2.degree. C. A high temperature region 330 is stretched covering all nozzles 340. In this way, all of the nozzles 340 are maintained at substantially uniform temperature.
However, because the nozzles 340 are aligned in a X direction, that is, in a direction parallel to head moving direction, the above-descried print head can print only on a relatively small region of a printing medium while reciprocating each time.
To overcome this problem, the present inventor has proposed a full-color print head having four nozzle heads, each formed with a plurality of nozzles aligned in the Y direction. Each of the nozzle heads ejects one of different color ink. The nozzle heads are mounted on a front panel which is formed with four channels each supplying ink to a perspective nozzle head. However, this type of nozzle head needs a heating device which has a certain length in the Y direction as well as X direction. Therefore, the conventional heating device described above cannot be adapted thereto.